Negative feed-back amplifier



Oct. 26, 1965 F. F. OFFNER NEGATIVE FEED-BACK AMPLIFIER Original FiledFeb. 2. 1959 m l l .P E N M v ,EN n wN l W .v n QN NV .GNN W mm um n dl@ A N Q @N www# Q T m wi #N l l I .uhu Y m m M A MWL .E M Q w N en |I:..lll uw .W Ewolmm m m Jl| \WN n@ N o m mw Franklin F. ffner- BY m,.@PCWPMEW f ATTORNEYS United States Patent O 3,214,703 NEGATIVEFEED-BACK AMPLIFIER Franklin F. Olner, 1890 Telegraph Road, Deerfield,Ill. Continuation of application Ser. No. 790,498, Feb. 2, 1959. Thisapplication July 14, 1964, Ser. No. 382,568 4 Claims. '(Cl. 3313-9) Thepresent invention relates to e-lectronic amplifiers and moreparticularly to an improved arrangement for providing negative feed-backfrom the output to the input sides of the amplifier in order tostabilize operation of the amplifier. The present application is acontinuation of my copending application Serial No. 790,498, filedFebruary 2, 1959 now abandoned.

A more particular object of the invention is to provide an improvedamplifier which includes an input modulator in the form of a chopper andan input transformer the primary of which is connected to the chopper.Alternating current negative feed-back from the output transformer isfed in series with the primary of the input transformer to provide astabilized operation. This form of feed-back has the advantage in thatby including the primary win'ding of the input .transformer in thefeed-back loop it tends to nullify effects of variations incharacteristics of the input transformer and it a-lso raises the inputimpedance to the amplifier.

The foregoing as well as other objects and advantages inherent in theinvention will become more apparent from the following detaileddescription of two embodiments thereof and from the accompanyingdrawings wherein:

FIG. 1 is a circuit diagram illustrating one embodiment of the inventionwherein the input transformer has a center .tapped primary and thealternating current negative feed-back is obtained by two separatetertiary windings in the output transformer which are connectedrespectively in ser-ies with the two halves of the primary winding ofthe input transformer; and

FIG. 2 is also a circuit diagram showing a second embodiment of theinvention wherein the input transformer has two separate primarywindings and the alternating current negative feed-back is obtained by asingle, center `tapped tertiary winding on the output transformer, thetwo halves of the tertiary winding being connected respectively inseries with the two separa-te primary windings of the input transformer.

With reference now to FIG. 1, the signal input which may, for example,be derived from electrodes placed on the head of a subject for recordingan electroencephalogram, is applied to input leads 1 and 2. Input lead 1is connected to the movable blade 3 of a chopper type switch 4, theblade 3 being oscillatable between two stationary contact members 5, 6.Oscillation of blade 3 is effected |by means of a vibrator coil 7 whichcan be supplied with alternating current at a suitable frequency, forexample, 400 cycles/sec. from a power source 8. Input lead 2 isconnected to the center tap point 9a on the primary winding 9 of aninput transformer 10. As blade 3 oscillates between contact members 5and 6 the input signal is applied in alternation to the two halves 9b,9c of the primary winding 9. A differential action is achieved throughuse of the chopper switch 4 in conjunction with the input transformer10. This results in isolation of the input circuit from ground, so thatthe amplifier is responsive essentially only to the voltage differencebetween input leads 1 and 2.

The output from the secondary winding 11 of input transformer is appliedas the input to the input stage of a multi-stage pre-amplifier unit 12which is shown in block form only since its details are not essential tothe invention itself as hereinafter defined. The output stage ofamplifier unit 12 is connected to the primary winding 3,214,703 PatentedOct. 26, 1965 rice 13 of an output transformer 14 and the latter isprovided with a secondary winding having a center tap point 15a andtwo-so-called tertiary windings 16 and 17 preferably having the samenumber of turns and which are preferably shielded from the primary andsecondary windings 13, 15 by an electrostatic shield member 18 which isgrounded. Tertiary winding 16 is connected back to the primary windingof the input transformer so as to be in series opposition to the voltageimpressed across primary winding half 9b when chopper blade 3 engagescontact member 5, and similarly, tertiary winding 17 is connected back-to the input transformer so as to be in series op position to thevoltage impressed across primary winding half 9c when chopper lblade 3engages contact member 6.

The two tertiary windings 16, 17 thus supply negative alternatingcurrent feed-back to the input of the amplifier and which the advantageof including the input transformer witfhin the feed-back loop. This:tends to nullify any adverse effects which may arise as a result ofvariations in input transformer characteristics and, in addition,provides a higher input impedance to the amplifier unit 12. This is dueto the effects of transformer losses and limited inductance beingdegenerated out by the feed-back.

In the illustrated embodiment, demodu'lation means are provided toreform the same wave form as was applied to the input to the amplifier,but in amplified form. The demodulator may take several forms but isillustrated as a chopper switch 20 having a contact blade 21 which isoscillatable between two stationary contact members 22, '.23. Contactblade 21 operates in synchronism with contact blade 3 of the inputchopper switch 4 and this is indicated schematically on the drawing bythe broken line 24 interconnecting the two chopper switches. The centertap 15a on the output transformer is connected to an output lead 25 andthe two outer ends of the secondary winding halves 15b, 15e areconnected respectively to the stationary contact members 22, 23 of thedemodulating chopper switch. The contact blade 21 of the latter can beconnected to ground, as indicated.

The amplified output at lead 25 can be used directly, or if desired, canbe put through a furtherI power amplifier unit, not illustrated, andthen fed to the point of use such as, for example, a recorder. For someapplications, the demodulator means can be omitted and the output fromthe secondary of the output transformer used directly.

In the embodiment illustrated in FIG. 2, the arrangement is generallysimilar to that shown in FIG. 1 and corresponding components have beenassigned the same reference numerals. However, the input transformer 26is arranged somewhat differently, the primary being split into twoseparate windings 26a, 26b, and the output transformer is provided witha center-tap tertiary winding, the center tap being indicated at 27 andthe winding halves by 28a, 28b. Potentiometer resistances 29 and 30 areconnected respectively in circuit with the winding parts 28a, 28b forpurposes of gain control, the adjustable taps on these resistances beingactuated simiultaneously by means of the schematically indicated linkage31 in order to adjust the amount of the negative feed-back alternatingvoltage which is to be connected in series with the primary windingparts 26a, 26b of the input transformer.

I claim:

1. In an amplifier of the commutated input type, the combinationcomprising an input transformer having a primary composed of first andsecond winding parts and a secondary winding, alternating currentamplifying means having its input connected to said secondary winding,means deriving first and second alternating current negative feed-backsignals from the output of said amplifying means, an input `signalcommutating means alternately connecting a signal source to said rst andsecond parts of said primary winding, circuit means connecting saidiirst alternating current negative feed-back signal in series with saidfirst part of said primary winding, and circuit means connecting saidsecond alternating current negative feedback signal in series with saidsecond part of said primary winding.

2. An amplifier of the commutated input type as de- ;{ined in claim 1wherein said means deriving said first and second' alternating currentnegative feed-back signals are constituted respectively by `outputs fromtertiary windings of an output transformer connected to the output ofsaid amplifying means.

3. An amplifier of the commutated input type as delined in claim 2wherein said irst and second primary winding parts of said inputtransformer are the two parts of an intermediately tappedsingle windingand said tertiary windings are separatefrom each other.

4. An amplifier of the commutated input type as dened in claim 2 whereinsaid first and second primary winding parts of said input transformerare separate from each other and said tertiary windings are constitutedrespectively by the two parts of an intermediately tapped singlewinding.

References Cited by the Examiner UNITED STATES PATENTS 2,901,563 8/59McAdam et al. 330-9 15 ROY LAKE, Primary Examiner.

1. IN AN AMPLIFIER OF THE COMMUTATED INPUT TYPE, THE COMBINATIONCOMPRISING AN INPUT TRANSFORMER HAVING A PRIMARY COMPOSED OF FIRST ANDSECOND WINDING PARTS AND A SECONDARY WINDING, ALTERNATING CURRENTAMPLIFYING MEANS HAVING ITS INPUT CONNECTED TO SAID SECONDARY WINDING,MEANS DERIVING FIRST AND SECOND ALTERNATING CURRENT NEGATIVE FEED-BACKSIGNALS FROM THE OUTPUT OF SAID AMPLIFYING MEANS, AN INPUT SIGNALCOMMUTATING MEANS ALTERNATELY, CONNECTING A SIGNAL SOURCE TO SAID FIRSTAND SECOND PARTS OF SAID PRIMARY WINDING, CIRCUIT MEANS CONNECTING SAIDFIRST ALTERNATING CURRENT NEGATIVE FEED-BACK SIGNAL IN SERIES WITH SAIDFIRST PART OF SAID PRIMARY WINDING, AND CIRCUIT MEANS CONNECTING SAIDSECOND ALTERNATING CURRENT NEGATIVE FEED-BACK SIGNAL IN SERIES WITH SAIDSECOND PART OF SAID PRIMARY WINDING.